Roosting behavior and group decision making in 2 syntopic bat species with fission–fusion societies

In many social species, individuals make group decisions to coordinate their actions. Despite the importance of group decisions for successful group living, few studies investigated how wild animals make group decisions in situations where group members have conflicting interests. This lack of empirical data is most evident for animal groups that regularly split into subgroups for some time. In groups with high fission–fusion dynamics, individuals can avoid group decisions that are not in their interest without foregoing benefits from being social. Here, we compare group decision making about communal day roosts in 2 syntopic bat species with a similar ecology and life history but a different fission–fusion behavior of their colonies. Daily roost monitoring during 3 breeding seasons showed that Bechstein’s bats formed subgroups 5 times more often than brown long-eared bats although both species occupied a similar number of bat boxes per colony and year. Bechstein’s bats were also significantly faster in discovering newly installed boxes and explored them further away from their established roosting areas compared with brown long-eared bats. In a field experiment where we created a conflict of interests among colony members where to roost, brown long-eared bats always achieved a colony-wide consensus about communal roosts. On the contrary, in Bechstein’s bats, individuals with conflicting interests often formed subgroups in different roosts according to their individual interests instead of reaching a consensus on a single communal roost. Our findings show that even ecologically similar species can use different group decision-making rules for solving an identical coordination problem.